There're more variables to the equation.
Does the VMS record to the live path and then archive elsewhere on other storage? If so, you will have up to 70Mbps writes and up to 70Mbps reads, simultaneously.
How much playback is expected (number of simultaneous clients and streams?)
Are you recording always or on motion/event?
If event based, estimated percentage of video that is actually recorded?
If event based, does the VMS allow you to buffer to RAM, or does it only prebuffer to disk?
I'm with Milestone, but our presales guy Jared Tarter is on here and would probably give more accurate numbers. My rule of thumb is a 7.2k rpm drive will not give more than 7MBps and a 15k rpm drive will not exceed 15MBps.
That's an over-simplification but I think it's in the ballpark. It also gets sticky when you start worrying about raid 5/6. Raid 0/1/10 scale relatively predictably.
The answers to the above questions will make a significant difference to the end result. The best thing to do is use the VMS vendor tools or services to build a spec ahead of time. Not only will it give you peace of mind, but speaking from experience it gives you tremendous strength if you are stuck dealing with what appears to be a performance problem after executing based on written recommendations.
SSD is a long story with varying schools of thought. If you use SSD for live storage, just make sure you are using enterprise grade SSDs. There's definitely a difference in both performance characteristics and longevity.